1. Field of Invention
The invention relates generally to wireless communication and more particularly to a system for notifying a communication application that a requested data rate and quality of service are not available.
2. Discussion of Related Art
Frequency Division Multiplexing (FDM) is a well known process by which multiple signals are modulated on different frequency carrier waves. FDM has been used for decades in radio and television broadcast. Radio and television signals are sent and received on different frequencies, each corresponding to a different “channel.”
Orthogonal Frequency Division Multiplexing (OFDM) has also been known in the art at least since the late 1960's. In OFDM, a single transmitter transmits simultaneously on many different orthogonal frequencies. Orthogonal frequencies are frequencies that are independent with respect to the relative phase relationship between the frequencies. In OFDM, the available bandwidth is subdivided into a number of equal-bandwidth “subchannels.” OFDM is advantageous for wireless communication because it has robust performance in multipath fading. In general, OFDM based systems do not require an equalizer for reliable communications. OFDM is employed in many standards used today for wireless communication. For example, both the IEEE 802.11a wireless LAN Standard and the 802.11g wireless LAN standard rely on an implementation of OFDM for signal transmission. The next generation 802.11n wireless LAN Standard, UWB wireless PAN Standard, and mobile WiMAX all use OFDM modulation for high rate communications. One early reference describing OFDM is R. W. Chang, Synthesis of band-limited orthogonal signals for multi-channel data transmission, Bell System Technical Journal (46), 1775-1796 (1966).
OFDM thus functions by breaking one high speed data stream into a number of lower-speed data streams, which are then transmitted in parallel (i.e., simultaneously). Each lower speed stream is used to modulate a subcarrier. This creates a “multi-carrier” transmission by dividing a wide frequency band (or channel) into a number of narrower frequency bands (or subchannels), each modulated with a signal stream. By sending multiple signal streams simultaneously, each at a lower rate, multipath or Raleigh fading can be reduced or eliminated without decreasing the overall rate of transmission.
OFDM is also implemented in a variety of communications systems occurring over wires. For example, OFDM is used in Asymmetric Digital Subscriber Line (ADSL) connections that adhere to the ITU G.992.1 standard. In the ADSL context, OFDM is sometimes referred to as Discrete Multitone Modulation, or DMT. OFDM is also often used to modulate signals to be transmitted over power wires. For example, the HomePlug powerline alliance established a standard for communication over power lines in a home. The HomePlug standard uses OFDM modulation.
Over the past several years, numerous network applications have come into common use that require certain transmission characteristics to work effectively. For example, Voice-over-IP (VoIP) communication, also known as IP Telephony, Internet Telephony, or Broadband Phone, is a telephone-like system that works entirely or partially over the Internet by converting a voice signal into a digital signal that can be transmitted in Internet packets. VoIP traffic requires a minimum data transfer rate to be useable for communication. Similarly, the transmission of video content (for example, video teleconferencing or streaming multimedia) over a network typically requires certain minimum transmission characteristics to be viewable.
When the Internet was first created, there were no systems in place to insure minimum transmission characteristics. Over time, standards for Quality of Service (QoS) have developed to provide guaranteed transmission characteristics such as minimum performance or throughput, or maximum latency. QoS can be implemented within a local network or over a broader area. One reference discussing QoS is the IEEE 802.1p standard, which is implemented at the media access control (MAC) framing layer on the protocol stack.